staging: omap-thermal: fix return value

Return the proper error value in _omap_bandgap_read_threshold.

Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>